As is generally recognized in the art, nickel-base alloys are used for the purpose of resisting the ravages occasioned by various corrodents. Notable in this regard are the nickel-chromium-molybdenum alloys as is set forth in the Treatise "Corrosion of Nickel and Nickel-Base Alloys", pages 292-367, authored by W.Z. Friend and published by John Wiley & Sons (1980). Among such alloys might be mentioned INCONEL.RTM. alloy 625, INCOLOY.RTM. alloy 825, Alloy C-276, Multiphase.RTM. alloy MP35N, HASTELLOY.RTM. alloys C, C-4 and the recently introduced alloy C-22.RTM..
Alloys of the type mentioned above are exposed to service conditions where, inter alia, severe crevice and pitting corrosion are encountered as well as general corrosion. Representative of such situations would be (a) pollution control applications, e.g., flue gas desulfurization scrubbers for coal fired power plants, (b) chemical processing equipment such as pressure vessels and piping, (c) the pulp and paper industry, (d) marine environments, particularly sea water, (e) oil and gas well tubing, casings and auxiliary hardware, etc. This is not to say that other forms of corrosive attack do not come into play under such operating conditions.
In endeavoring to develop a highly useful and practical alloy for the above applications/service conditions, there seems to have been an emphasis in the direction of using chromium and molybdenum levels as high as possible, and often together with tungsten. (See, for example, Table I below which gives the nominal percentages of various well known commercial alloys.)
TABLE I ______________________________________ Alloy Cr plus Mo plus W ______________________________________ Alloy 625* 21.5 Cr + 9 Mo C-276* 15.5 Cr + 16 Mo + 3.75 W MP35N* 20 Cr + 10 Mo C* 15.5 Cr + 16 Mo + 3.75 W C-4* 18 Cr + 15.5 Mo C-22 22 Cr + 13 Mo + 3 W X* 22 Cr + 9 Mo + 0.6 W ______________________________________ *Page 296 of W. Z. Friend treatise: Note Co, Cb, Ta, etc. are often found in such materials.
While high chromium, molybdenum and tungsten would be desirable, it can also give rise to a morphological problem, to wit, the formation of the Mu phase, a phase which forms during solidification and on hot rolling and is retained upon conventional annealing. There is perhaps not complete agreement as to what exactly constitutes Mu phase, but for purposes herein it is deemed to be appreciably a hexagonal structure with rhombohedral symmetry phase type comprised of (Ni, Cr, Fe, Co, if present,).sub.3 (Mo, W).sub.2. P phase, a variant of Mu with an orthorhombic structure, may also be present.
In any case, this phase can impair the formability and detract from corrosion resistance since it depletes the alloy matrix of the very constituents used to confer corrosion resistance as a matter of first instance. It is this aspect to which the present invention is particularly directed. It will be observed from Table I that when the chromium content is, say, roughly 20% or more the molybdenum content does not exceed about 13%. It is thought that the Mu phase may possibly be responsible for not enabling higher molybdenum levels to be used where resistance to crevice corrosion is of paramount concern.
The foregoing aside, in striving to evolve the more highly corrosion resistant alloy, other considerations must be kept in focus. That is to say, corrosion resistance notwithstanding, such alloys not only must be hot workable but also cold workable to generate required yield strengths, e.g., upwards of 689 to 862 or 1035 MPA, together with adequate ductility. In addition, alloys of the type under consideration are often subjected to a welding operation. This brings into play corrosive attack at the weld and/or heat-affected zones (HAZ), a problem more pronounced where elevated operating temperatures are encountered, e.g., in the chemical process industry. Without a desired combination of mechanical properties and weldability an otherwise satisfactory alloy could be found wanting.